After an oil well has been partly drilled and suspected producing formations have been penetrated, it is necessary to make various tests to determine production possibilities of various formations. One of the test techniques involves the use of a tool which is known as a formation tester. An exemplary formation tester is set forth in U.S. Pat. Nos. 4,375,164 and 4,593,560 assigned to assignee of the present disclosure. As set forth in those disclosures, the tool is adapted to be lowered into the well bore, supported on an armored logging cable enclosing certain conductors for providing surface control for the tool. The logging cable extends to the surface and passes over a sheave and is spooled on a reel or drum. The conductors in it connect with suitable surface located power supplies, controls, and recorder. The formation tester is lowered to a specified depth in a well. At that elevation, a backup shoe is extended on one side of the formation tester and formation testing apparatus is extended diametrically opposite into the formation of interest. The equipment so extended normally includes a surrounding elastomeric sealing pad which encircles a smaller extendible snorkel which penetrates a formation as the formation will permit, up to a specified depth. The snorkel is ideally isolated from fluid and pressure in the well to be able to test the formation. The snorkel is extended into the formation to enable direct fluid communication from the formation into the tool. Moreover, it is isolated from invasion of the well borehole fluid and pressures therein to permit a pressure sensor to obtain formation pressure. Further, a sampling chamber elsewhere in the formation tester can be selectively connected through the snorkel by suitable valves to obtain delivery of a fluid sample from the formation. The fluid sample typically may include a relatively small sample which is a pretest sample, and if that is acceptable, a larger sample can be drawn through the snorkel. Various pretest and sample volumes are selected and determined under control from the surface.
Testing procedures require a substantial interval. For instance, isolation steps must be undertaken to assure that the formation tester properly obtains data from a single formation without invasion of other well fluids from different strata. These procedures involve extension and retraction of the packer and snorkel described above. These steps are normally accompanied by the extension of certain backup shoes which set backup shoes on the opposite side of the formation tester in the borehole Thus, the references noted above describe apparatus which extends the snorkel on one side of the tool body and which extend backup pistons on the opposite side to assure that adequate force is delivered to position the snorkel in the formation of interest.
Ideally, test procedures are conducted as rapidly as possible to assure that the tests are conducted at a minimum cost in rig time.
One of the steps carried out by the formation tester is extension of the snorkel and surrounding pad which achieves a seal to isolate the formation. Additionally, backup pistons are extended, thereby assuring that backup shoes are anchored in the well borehole.
The present invention is directed to an improved system including a hydraulic circuit within the formation tester which assures that the packer surrounding the snorkel maintains a proper seal and is not subjected to formation erosion in unconsolidated formations. Assume that a sample is to be taken form an unconsolidated formation. A probe is extended on the end of a snorkel into that formation. The formation is held in place by a surrounding elastomeric seal confronting the formation. The probe enters the formation, begins production of formation fluids through the probe, and continues for an interval. In the immediate zone near the snorkel, loss of fluid may cause sloughing of the formation which may erode in the vicinity of the seal contact. If this occurs, fluid at higher pressures within the well borehole may cause very substantial, almost instantaneous erosion around that seal, and flood the region near the snorkel probe and thereby damage the formation, potentially mixing with the sample taken through the probe and causing problems with formation tester withdrawal. The seal is thus undercut by erosion, and raising doubts about the integrity of the sample captured from that particular formation.
A related problem which arises from unconsolidated formations is the abrasive nature of formation particles (typically sand-like particles) which are produced. The connate formation fluids will carry particles through the probe and into the formation tester. When this occurs damage directly results to the various valves in the hydraulic system for control of the formation tester. The abrasive particles with the formation fluid typically will erode the valve seats or valve elements of valves in the hydraulic circuitry. This erosion problem can create difficulties. For instance, once the combination of valve element and valve seat has been eroded to the degree where leakage occurs across the seat, then either incorrect pressure measurements will be obtained or improper filling of the sample container in the formation tester might occur. In any event, repairs are difficult ordinarily to implement and the formation tester must be repaired.
The present apparatus is directed to a formation tester which accomplishes sampling where the sample is obtained through two valves in series. The first valve serves an entirely different function than the second valve. The two valves are used jointly to prevent problems as described above. A first valve is serially connected with the probe on the snorkel. The sample is introduced to the first valve and sample pressure is detected by a pressure sensor. A motorized valve equipped with a hard seat in conjunction with the valve element is adjusted in position so that a choking function of fluid flow is accomplished. The pressure sensor provides an uplink signal along the logging cable to the surface so that pressure is known. Formation pressure foreknowledge enables preliminary setting of the first valve by means of a downlink signal sent to a motor controller which adjusts the opening of the first valve. This preliminary adjustment prevents large differential pressures from occurring across the face of the seal around the snorkel and thereby reduces, indeed even prevents, erosion of the formation sand around the elastomeric snorkel seal. The first valve can be opened to different choking positions to thereby change the flow rate in part dependent on the pressure measurement obtained by the pressure sensor. Since a complete seal is not required of the first valve, maintenance is reduced because it is not required to close on fluid flow which is laden with abrasive particles. A second serial valve is included. It is driven by a solenoid so that it switches between open and closed positions. The second valve opens into the sample storage chamber. This valve is constructed with a soft seat arrangement. It is open and closed in a binary fashion so that the storage chamber can be easily isolated. In the event that closure on abrasive particles in the fluid flow produces damage, it is preferably constructed so that the soft wearing component is easily removed and service can be quickly accomplished. Accordingly, the first and second valves arranged in series provide protection of the valving system.
So that a more complete understanding of the present apparatus can be obtained, attention is directed to the detailed description below, which, when considered in conjunction with the drawings, sets forth the present apparatus in substantial detail.